Physical vapor deposition in a vacuum environment is a commonly used way of depositing thin organic material films, for example in small molecule OLED devices. Such methods are well known, for example Barr in U.S. Pat. No. 2,447,789 and Tanabe et al. in EP 0 982 411. The organic materials are often subject to degradation when maintained at or near the desired rate-dependent vaporization temperature for extended periods of time. Exposure of sensitive organic materials to higher temperatures can cause changes in the structure of the molecules and associated changes in material properties.
The organic materials used in OLED devices have a highly non-linear dependence of vaporization rate on source temperature. A small change in source temperature leads to a very large change in vaporization rate. Despite this, prior art devices employ source temperature as the only way to control vaporization rate. To achieve good temperature control, prior art deposition sources typically utilize heating structures whose solid volume is much larger than the organic charge volume, composed of high thermal-conductivity materials that are well insulated. The high thermal conductivity insures good temperature uniformity through the structure and the large thermal mass helps to maintain the temperature within a critically small range by reducing temperature fluctuations. These measures have the desired effect on steady-state vaporization rate stability but have a detrimental effect at start-up. It is common that these devices must operate for long periods of time (e.g. 2-12 hours) at start-up before steady state temperature distribution and hence a steady vaporization rate is achieved. It is also common that these devices also require long times to cool down, and thus significant amounts of organic material, some of which can be expensive or difficult to synthesize, can be lost. Furthermore the steady state slowly drifts as material is consumed from the sources, and input power must be changed (in order to alter the temperature distribution) to maintain a constant vaporization rate.
The current method to minimize material time at high temperature and to maximize machine operation time by minimizing the start-up and cool-down times of the material-containing sources requires using duplicate sources of the same material sequentially. For example, rather than using one source continuously for eight days, two sources can be used for four days each or eight sources can be used in a serial process for one day each by overlapping the start-up and cool-down times. Duplicate sources, however, increase equipment size and cost, especially if the number of duplicate sources or the number of materials that require duplicate sources is large.
Forrest et al. (U.S. Pat. No. 6,337,102 B1) disclose a method for vaporizing organic materials and organic precursors and delivering them to a reactor vessel wherein the substrate is situated, and delivery of the vapors generated from solids or liquids is accomplished by use of carrier gases. The organic materials are held at a constant temperature that is high enough to saturate the incoming carrier gas at all possible flow rates. Deposition rate is controlled by adjusting the carrier-gas flow rate. In one embodiment of their invention, Forrest et al. locate the substrates within a suitably large reactor vessel, and the vapors carried thereto mix and react or condense on the substrate. Another embodiment of their invention is directed towards applications involving coating of large area substrates and putting several such deposition processes in serial fashion with one another. For this embodiment, Forrest et al. disclose the use of a gas curtain fed by a gas manifold (defined in the disclosure as “hollow tubes having a line of holes”) in order to form a continuous line of depositing material perpendicular to the direction of substrate travel.
One major problem in the approach disclosed by Forrest et al. is that all of the materials are continuously heated in high thermal mass systems to maintain tight temperature control. This exposure to high temperatures for extended periods of time increases the likelihood of degradation of some materials in the same way as the methods taught by Barr and Tanabe et al. Another problem in the approach disclosed by Forrest et al. is that cool-down and start-up times to reload material are long, due to the high thermal mass of the system and the requirement that all materials be at a uniform temperature before starting the carrier gas flow.
Also known in the art are systems such as taught by Hoffman et al. of Applied Films GmbH & Co. in their paper from the Society for Information Display 2002 International Symposium, SID Digest 02 pp. 891-893. These systems combine large heated remote sources similar to the type used by Barr and Tanabe et al. with manifolds to distribute the material vapor. These systems suffer from the same problems as the methods taught by Barr, Tanabe et al., and Forrest et al. with respect to material degradation, due to long term exposure to high temperatures and long cool-down and start-up times due to the high thermal mass of the heating system.
The approaches to vapor delivery as disclosed by Forrest et al. and Hoffman et al. can be characterized as “remote vaporization” wherein a material-is converted to vapor in an apparatus external to the deposition zone and more likely external to the deposition chamber. Organic vapors, alone or in combination with carrier gases, are conveyed into the deposition chamber and ultimately to the substrate surface. Great care must be taken using this approach to avoid unwanted condensation in the delivery lines by use of appropriate heating methods. This problem becomes even more critical when contemplating the use of inorganic materials that vaporize to the desired extent at substantially higher temperatures. Furthermore, the delivery of the vaporized material for coating large areas uniformly requires the use of gas manifolds.
Current remote-vaporization methods suffer from the problems of long material exposure to high temperatures and start-up and cool-down delays due to high thermal mass heating systems; however, these systems have some advantages over the methods taught by Barr and Tanabe et al. with respect to coating uniformity and control of instantaneous deposition rates. Although these remote vaporization methods can stop deposition fairly quickly by closing valves for the carrier gases in the method of Forrest et al. or for the organic vapors in the method of Hoffman et al., the organic vapors and carrier gases downstream of the valves will continue to exit the manifold until the manifold pressure drops to the deposition chamber pressure. Likewise this method can start deposition fairly quickly but organic vapors and carrier gases will not reach steady state deposition rates until the manifold has reached steady state pressure. This is a problem due to remote vaporization combined with structures, such as valves, to control the flow of organic vapors that are also remote from and not contiguous to the manifold. These remote structures do not quickly control the passage of organic material through the manifold apertures, resulting in delays in starting and stopping deposition. Remote vaporization systems with remote valves do not resolve the significant issue of long start-up and cool-down times for loading fresh material, due to the high thermal mass of these systems, nor do they resolve the major issue of material degradation due to extended exposure to high temperature in these systems.
Furukawa et al., in Japanese Unexamined Patent Application 9-219289, disclose a method of forming an organic thin-film electroluminescent element by a flash vapor deposition method. While this method can start and stop quickly, it cannot be run as a continuous process as taught by Furukawa et al. The organic material is dropped onto a heated plate. Furakawa is silent on the nature of the powder delivery system, and how it assures that the desired quantity of powder is actually dropped on the heated plate, and therefore how vaporization rate, the deposited film thickness, and thickness uniformity are controlled. Also unclear is how the powder delivery system, with a temperature below the condensation temperature of the just-created vapor, is prevented from acting as a cold finger upon which a portion of the just-created vapor condenses.